Vapor compression systems, such as heat pumps, refrigeration and air-conditioning systems, are widely used in industrial and residential applications. The introduction of variable speed compressors, variable position valves, and variable speed fans to the vapor compression cycle has greatly improved the flexibility of the operation of such systems. It is possible to use these new components to improve the efficiency of vapor compression systems by controlling the components correctly.
For example, a speed of the compressor can be adjusted to modulate a flow rate of a refrigerant. The speed of an evaporator fan and a condenser fan can be varied to alter heat transfer coefficients between air and heat exchangers. The change in an expansion valve opening can directly influence a pressure drop between the high-pressure side and the low-pressure side of the vapor compression system, which, in turn, affects the flow rate of the refrigerant, as well as superheat at the corresponding evaporator outlet.
Additionally, vapor compression systems are becoming increasingly complex. Integrated systems with multiple indoor units connected to a single outdoor unit are common in residential applications, and some commercial applications employ vapor compression systems with multiple outdoor units and multiple indoor units, all under the direction of a single control system. It is understood that each unit mentioned above includes of a heat exchanger and may include a variable speed fan and/or a variable position expansion control device. Therefore, these complex systems are characterized by a large number of actuators. It is desirable to operate the system where each actuator is controlled such that energy consumption is optimized.
Vapor compression systems can consume large quantities of electrical energy, and therefore, incur a large operating cost. Accordingly, it is desired to determine control inputs that optimize a performance of the vapor compression system. One performance characteristic is optimal energy consumption. A number of methods for controlling operations of the vapor compression system are disclosed in the art.
For example, one method for controlling a vapor compression system, disclosed in U.S. Pat. No. 5,735,134, considers the possibility of sudden change in environmental or thermal load requirements, monitors the vapor compression system in real-time, and determines, based on these actual real-time measurements, a set of parameters to enable the system to operate at maximum coefficient of performance. However, that method is time consuming, and requires substantial real time computational resources and a mathematical model of the vapor compression system.
Another method, disclosed in U.S. Pat. No. 7,076,962, first determines an amount of thermal flow across an evaporator or a condenser. Next, the amount of thermal is used to determine a set of optimal control inputs. As the amount of thermal flow is directly related to the operation of the vapor compression system, this determination is difficult to avoid. However, that approach is not optimal because nonlinear phenomena do not substantially affect the efficiency of the components of the vapor compression system.
Another method, disclosed in U.S. Pat. No. 7,246,500, reduces energy consumption of the system by modulating the speed of a condenser fan of the vapor compression system. However, that method is suboptimal because other components of the system are not adjusted. Thus, a combination of operational parameters of various components of the vapor compression system is not optimized.
Yet another method, disclosed in JP 2000-257941, reduces energy consumption of cold water or hot-water in the air conditioner by measuring the room temperature with a thermometer and retrieving a value of a valve opening from a valve opening table using the room temperature as an index. However, conventional vapor compression systems typically have a number of different components, including but not limited to the valve, which need to be controlled concurrently. Moreover, that method determines the valve opening based on outside environment conditions only, which is not always optimal.
Accordingly, there is a need in the art for a control system and a method for controlling operation of the vapor compression system such that thermal load of the operation is met and a performance of the system is optimized.
Conventionally, methods maximizing energy efficiency rely on the use of mathematical models of the physics of vapor compression systems, as described, e.g., in U.S. Pat. No. 5,735,134. Those model-based methods attempt to describe the influence of commanded inputs of the components of the vapor compression system on the thermodynamic behavior of the system and the consumed energy. In those methods, models are used to predict the combination of inputs that both meets the thermal load requirements and minimizes energy.
However, the use of mathematical models for the selection of optimizing inputs has several important drawbacks. Firstly, models rely on simplifying assumptions to produce a tractable representation, and those simplifications are especially required for multi-unit vapor compression systems with complex physical interactions and numerous system actuators. Those assumptions often ignore important effects or do not consider installation-specific characteristics such as room sizes, causing the model of the system to deviate from an actual operation of the system.
Secondly, variations during the manufacturing process of those systems are often so large as to produce vapor compression systems of the same type that have different input-output characteristics, and therefore a single model cannot accurately describe the variations in the characteristics.
Thirdly, those models are difficult to derive and calibrate. For example, parameters that describe the operation of a component of a vapor compression system, e.g., a compressor, are experimentally determined for each type of the compressor used, and a model of a complete vapor compression system may have many such parameters. Determining the values of these parameters for each model is a difficult process.
Finally, vapor compression systems are known to vary over time. A model that accurately describes the operation of a vapor compression system at one time may not be accurate at a later time as the system changes, for example, due to slowly leaking refrigerant, or the accumulation of corrosion or debris on the heat exchangers.
FIG. 1 shows a conventional multi-unit vapor compression system 100 that includes components, e.g., variable setting actuators. Often, the multiple units are intended to allow the independent regulation of corresponding multiple room or zone temperatures. In the case where the vapor compression system is operated in a cooling mode, as shown in FIG. 1, heat is removed from the indoor units to the outdoor units and therefore the indoor units act as evaporators. Conversely, when operated in heating mode, which is not shown, heat is added to the indoor units from the outdoor units and the indoor units are act as condensers.
The components can include an evaporator fan 114, a condenser fan 113, an expansion valve 111, and a compressor 112. The system can be controlled by a supervisory controller 120 responsible for accepting setpoints 115, e.g., from a thermostat, and readings of a sensor 130, and outputting a set of control signals for controlling operation of the components. The supervisory controller 120 is operatively connected to a set of control devices for transforming the set of control signals into a set of specific control inputs for corresponding components. For example, a supervisory controller is connected to a compressor control device 122, to an expansion valve control device 121, to an evaporator fan control, device 124, and to a condenser fan control device 123. Also, it is possible to connect multiple heat exchangers to independently regulate multiple zone temperatures. Shown in FIG. 1 is a set of N evaporators in the vapor compression system. Each of these N evaporators may include corresponding evaporator fans 117, and those fans may be operatively connected to a corresponding fan control devices 126.
The multiple evaporators illustrate one of many configurations of multiple unit vapor compression systems. Other configurations of the vapor compression system may include multiple expansion valves, with, for example, one or more expansion valves for each evaporator. These multi-unit systems contain a large number of actuators, and it is desirable to operate the system so that each actuator is operated such that the overall system consumes minimum energy.
However, the operation of the system can be not optimal. In consideration of the above, there is a need in the art for a method for controlling operation of the vapor compression system such that heat load of the operation is met and a performance of the system is optimized, where the method is able to control many actuators concurrently, is not model-based, and can adapt over time as the system characteristics evolve.